Specific gravity or the solids content of potatoes is an important determinant of harvest quality. A processing potato must have a high specific gravity and low sugar content. A high specific gravity results in french fries that have a more desirable mealy texture and flavour, are crisp and absorb less oil during frying compared to fries from potatoes with a lower dry matter content. Determining the specific gravity is necessary because it indicates how much water must be evaporated from the potato during the dehydration process prior to frying the strips. It is actually a measure of the dry matter or “solids” in the potato. High specific gravity potatoes make the best French fries and dehydrated potato products. In practice, this attribute of a tuber is an indicator of maturation that the industry uses as a reference to judge fry quality, baking characteristics and storability. More importantly the specific gravity measurements reflect environmental factors and cultural management procedures that were made during the production season. In addition, the distribution of starch or dry matter, sugar content, types of sugars and distribution, internal cell structures, tuber size and shape, tuber defects such as growth cracks and hollow heart can also influence specific gravity measurements. Improper sampling and methods of determinations could mislead specific gravity measurements and result in improper equipment settings on the processing line. This results in product quality losses for the processor.
There is no product or device in existence that can perform automatic monitoring of specific gravity on the french fry processing line. Specific gravity is currently determined by a crude manual method. A sample of tubers and several tubs of water (with salt added to make brine at different specific gravities) are used. The weight in air versus the weight in water method is another of the common methods of specific gravity determinations. Selected sample units are first weighed in air and then the same unit is re-weighed suspended in water. Specific gravity can then be calculated using the following formula:Specific gravity=Weight in air/(Weight in air−Weight in water)
This is a time-consuming testing method that does not allow for quick and accurate sampling and therefore it does not allow the processor to set the fryers and drier machines in real time. Both sampling methods are also not very accurate as contamination of the brine barrel with dirt, potato starch etc. and contamination of the weighing water occurs after only a few samples have been tested.
The other common method of measuring specific gravity is to use a potato hydrometer. The hydrometer consists of a float with the neck graduated to specific gravity readings. A basket containing the sample is hung beneath the float and the whole assembly placed in water. After some time the float remains steady and the specific gravity is read from where the water level is on the neck of the hydrometer. The higher the specific gravity, the deeper the hydrometer will be in the water.
The disadvantage of this method is that the hydrometer is calibrated to a fixed weight of potatoes in the basket and therefore the sample placed in the basket must be exactly this specified weight.
It can be time consuming finding tubers of the right size to make the exact weight, and also the hydrometer can bob up and down for some time before a reading can be made. If the hydrometer is knocked about, the chart of specific gravity readings inside the neck can be moved, thereby resulting in totally inaccurate readings. Hence, this method, although commonly used, is not thought to be very accurate. A further disadvantage is the sample size: the hydrometer is limited to the amount specified in the basket and therefore is not a very representative sample.